It is particularly difficult to cast casings made of light alloy, that is to say of an aluminum- or magnesium-based alloy, and containing pipes made of a similar alloy. Techniques commonly used consist either in casting the pipe at the same time as the casing or in integrating a pipe into the mold prior to casting, but ensuring that said pipe is not damaged by the molten metal.
In the first case, a mold is produced, said mold forming a cavity, for example by means of a sand core, corresponding to the pipe to be produced; this technique has the advantage that the pipe can be produced from the same alloy as the casing and thus subsequent expansions of the casing are dealt with and so the occurrence of stresses which would arise on account of the use of different materials is avoided. The casing mold is manufactured by fitting sand cores, which reproduce the interior of the pipe to be manufactured and which are positioned at the location at which this pipe is intended to appear. The drawback of this technique is that it is necessary to check the pipes after casting in order to ensure that the sand has been removed properly at the end of the operation and that there are no longer any grains, which could detach during operation and could cause faults in the operation of the mechanisms fitted inside the casing. This verification, in conjunction with possible alterations, is a long and expensive operation which it is desirable to eliminate. Further drawbacks are also encountered, such as creeping of the sand cores, which creates imprecision in the positioning of the outlets of the lines, such as variations in thickness along the pipes, or even such as the possible presence of voids at the intersections of the sand cores. Furthermore, the tubes produced directly during casting have greater thicknesses than conventional pipes which are produced independently. The overall mass of the casing is prejudiced even more, it being possible for the increase in mass to reach 10% of the blank, in other words of the casing straight after casting, before it is machined to the final dimensions.
Another method of manufacturing a casing made of light alloy is described in European patent application EP 0470021 from Montupet S. A. In this case, the pipes are produced independently of the casing and then integrated into the latter before the alloy is cast. However, this technique requires that a large number of precautions be taken, such as, for example, selecting the alloy of the pipe such that its melting point is higher than that of the alloy to be melted and/or providing that a cooling fluid flows through the pipe during casting. In addition, there must be a, very short, period of contact between the tubes and the molten metal.
Also known is document U.S. Pat. No. 4,450,886, which relates to a method of producing intake pipes in the automotive field, into which pipes an exhaust gas recirculation tube is integrated. This document does not mention in particular how the recirculation tube is held during the casting of the metal of the intake tube.